This invention relates to a method of perforating a web, and, more particularly, to a method of perforating a paper web which is used to form rolls of bathroom tissue or paper towels.
Rewinders are used to convert large parent rolls of paper into retail sized rolls of bathroom tissue and paper towels. Two types of rewinders are commonly used--center rewinders and surface rewinders. Center rewinders are described, for example, in U.S. Reissue Pat. No. 28,353 and wind the web on a core which is rotated by a mandrel to form a wound log. Surface rewinders are described, for example, in U.S. Pat. Nos. 4,723,724 and 5,104,055 and wind the web on a core which is rotated by a three roll cradle to form a wound log.
Before the web is wound into a log, the web is perforated along transverse lines which define the sheet length of the rolls of bathroom tissue or paper towels. After the log is wound, the log is cut into individual rolls by a log saw.
The traditional style perforator which is described, for example, in U.S. Pat. Nos. 2,870,840, Re. 26,418, and 3,264,921, consists of a rotating perforator roll and a stationary anvil bar. The perforator roll includes multiple rows of blades which extend parallel to the axis of the roll and which are spaced at a fixed interval around the circumference of the roll. The blades are notched to provide a specific perforation pattern depending upon the web. The stationary anvil bar is a large casting which holds a series of carbide anvils in a helical pattern arrangement. Each anvil can be adjusted radially to set the desired interference between the perforator blades and the anvils.
The perforation pattern is applied to the web as the web passes through the perforator by pinching the web between the perforator blades and the anvil blades. The entire width of the web is perforated in straight lines across the web at fixed intervals which define the perforation length or the sheet length of the rolls which will be formed by winding the web. The perforated web passes from the perforator to a rewinder to be wound into logs using either surface or center winding technology. Once the web is perforated, the cutting of the logs using the traditional style perforation pattern does not require precise log cutting because the perforation pattern is consistent across the web.
The problem with the present art is that the speed of the rewinder is often limited by web blowouts, where the web tension exceeds the tensile strength of the perforations. This can result in the web breaking along a perforation during winding. As rewinder speed is increased, tension can increase due to increased rewinder dynamics such as mandrel vibration.
In order to achieve a desired product diameter and firmness, the web tension is increased and may approach the tensile strength of the perforations. If the web tension meets or exceeds the tensile strength of the perforations, the web will tear on the perforations.
For some products, the tensile strength of the web is minimized in order to achieve a unique web characteristic. However, reduced web tensile strength will reduce the tensile strength of the perforated web.